Trickster's Tale: Hu Yan Zhuan

When half–fox demon, half-human Feijin loses his family at an early age, he is taken in by and becomes the pupil of Sage Gaochun. As the centuries pass, Feijin's admiration for his master turns romantic, but such relationships between a master and his pupil are not allowed. And though Feijin continues to confess his feelings to Gaochun, he is gently rebuffed each and every time. Desperate, Feijin puts his master under a spell and whisks him off to bed, but the typically gentle and reserved Gaochun surprises Feijin with his passion. It's only later that he learns his master had once been deeply in love with a demon and was severely punished for it!

(Source: SuBLime)